1. Field of the Invention
The present invention relates to an image display device capable of displaying different images for a plurality of viewpoints, and to a terminal device having the image display device loaded thereon. More specifically, the present invention relates to an image display device and a terminal device capable of displaying stereoscopic images of an excellent quality.
2. Description of the Related Art
There have been conducted studies on image display devices capable of displaying different images for a plurality of viewpoints. As an example, there is a stereoscopic liquid crystal display device which makes it possible to provide different displays for a plurality of viewpoints by using an optical element capable of separating images.
To achieve such function, there have been conducted studies on a large number of stereoscopic image display systems. This type of stereoscopic image display systems can be classified roughly into two types, i.e., one is a type that uses an eyeglass, and the other is a type that does not use any eyeglass. The type using an eyeglass includes a polarizing eyeglass type which utilizes polarization and an anaglyph system that utilizes differences in colors, etc.
In the meantime, each of the above-described systems essentially cannot avoid a troublesome work of wearing an eyeglass. Thus, there have recently been active studies conducted on the no-eyeglass system that does not require any eyeglass. As such no-eyeglass system, there are a parallax barrier type, a lenticular lens type, and the like. With these types, images with parallax are presented for both eyes on the left and right sides, respectively, so as to achieve a stereoscopic image display device.
The lenticular lens type of this no-glass system was proposed in 1910s by Ives et al. as depicted in “Three-dimensional display” Chihiro MASUDA, Sangyo Tosho Publishing Co., Ltd. (Non-Patent Document 1), for example. FIG. 28 shows an example of this lenticular lens 121. As shown in FIG. 28, the lenticular lens 121 has a plane on its one face, and a plurality of semicylindrical convex parts (cylindrical lenses) 122 extended along one direction are formed on the other face in such a manner the longitudinal directions thereof are in parallel to each other.
Further, FIG. 30 shows an optical model diagram regarding a stereoscopic image display method when using the lenticular lens 121. As shown in FIG. 30, the lenticular lens 121, a display panel 106, and a light source 108 are disposed in order from the viewer side, and pixels of the display panel 106 are located on the focal plane of the lenticular lens 121.
Referring to FIG. 30 mentioned above, pixels 123 for displaying images for right eye 141 and pixels 124 for displaying images for left eye 142 are alternately arranged in the display panel 106. In this case, the group of mutually neighboring pixels 123 and 124 correspond to each convex part 122 of the lenticular lens 121. With this, light emitted from the light source 108 and transmitted through each of the pixels 123 and 124 is distributed to directions towards the left and right eyes by the convex parts 122 of the lenticular lens 121. This enables the left and right eyes to recognize different images, thereby making it possible for the viewer to recognize stereoscopic images.
In the meantime, the parallax barrier type was conceived by Berthier in 1896, and it was substantiated by Ives in 1903. FIG. 29 mentioned above is an optical model diagram which shows a stereoscopic image display method using a parallax barrier. As shown in FIG. 29, a parallax barrier 105 is a barrier (shielding plate) on which a large number of thin stripe-type openings (that is, slits 105a) are formed. A display panel 106 is disposed in the vicinity of one of the surfaces of the parallax barrier 105.
In this display panel 106, right-eye pixels 123 and left-eye pixels 124 are arranged in the direction orthogonal to the longitudinal direction of the slits 105a. Further, a light source 108 is disposed in the vicinity of the other surface of the parallax barrier 105, i.e., on the opposite side of the display panel 106.
Among the light emitted from the light source 108, the light passed through the opening parts (slits 105a) of the parallax barrier 105 and then transmitted the right-eye pixels 123 turns out as light flux 181 as shown in FIG. 29. Similarly, among the light emitted from the light source 108, the light passed through the slits 105a and then transmitted the left-eye pixels 124 turns out as light flux 182. Here, the position of the viewer at which a stereoscopic image can be recognized is determined depending on the positional relation between the parallax barrier 105 and the pixels. That is, it is necessary for the right eye 141 of the viewer to be within a passing area of all the light fluxes 181 that correspond to a plurality of right-eye pixels 123, and for the left eye 142 of the viewer to be within a passing area of all the light fluxes 182.
This is a case where the middle point 143 of the right eye 141 and the left eye 142 of the viewer is located within a quadrilateral stereoscopy viewable area 107 shown in FIG. 29. A segment passing through an intersection point 107a of diagonal lines in the stereoscopy viewable area 107 is the longest segment among the segments extended in the arranging directions of the right-eye pixels 123 and the left-eye pixels 124 in the stereoscopy viewable area 107. Thus, when the middle point 143 comes at the intersection point 107a, the allowance for the position of the viewer to be shifted in the left and right directions becomes the maximum. Therefore, it is the most preferable position as the viewing position.
Accordingly, in this stereoscopic image display method, the distance between the intersection point 107a and the display panel 106 is defined as the optimum viewing distance OD (=S), and it is recommended to the viewer to view the images at this distance S. Note that a virtual plane in the stereoscopy viewable area 107 at which the distance from the display panel 106 is the optimum viewing distance S is referred to as an optimum viewing plane 107b. With this, the light from the right-eye pixels 123 and the left-eye pixels 124 is to reach the right eye 141 and the left eye 142 of the viewer, respectively. Therefore, the viewer can recognize the image displayed on the display panel 106 as a stereoscopic image.
At the time when the parallax barrier system is initially conceived, there was an issue of having an obstacle and low visibility since the parallax barrier was placed between the pixels and the eyes. Due to the recently achieved liquid crystal display devices, however, the visibility has been improved because it is possible to place the parallax barrier 105 in the back side of the display panel 106 as shown in FIG. 29. Thereby, there have now been studies actively conducted on the parallax-barrier type stereoscopic image display devices.
Note here that the above-described parallax barrier type is a type which “conceals” unnecessary light rays by the barrier, whereas the above-described lenticular lens type is a type which changes the traveling direction of the light. Thus, the latter type, i.e., the lenticular lens type, has such an advantage that there is no degradation in the brightness of the display screen theoretically. Because of this advantage, there has been considered application of the lenticular lens type to portable devices and the like where the high-luminance display and low-power consumption performance are taken into serious consideration. The conventional stereoscopic image display devices using the lenticular lenses use transmissive-type liquid crystal display devices as the display panels thereof.
Further, other than the stereoscopic image display device described above, a plural-image simultaneous display device which displays a plurality of images simultaneously is also developed as the image display device using the lenticular lens (see Japanese Unexamined Patent Publication 06-332354 (Patent Document 1), for example). This plural-image simultaneous display device is a display which displays different images for each of the viewing directions simultaneously with a same condition by utilizing an image distributing function of the lenticular lens.
This enables a single plural-image simultaneous image display device to provide different images with each other simultaneously to a plurality of viewers located at different directions from each other with respect to the display device. Patent Document 1 depicts that the use of this plural-image simultaneous display device makes it possible to decrease the setting space and the cost for the electricity compared to a case where it is necessary to prepare the number of normal single-image display devices for the number of images to be displayed simultaneously.
Further, the use of a reflective-type image display device having a reflection plate as a display panel has been investigated. This reflective-type image display device reflects light making incident from the outside by the reflection plate located inside the display device and utilizes the reflected light as a display light source. Thus, a back light or a side light as the light source is unnecessary.
In the meantime, the transmissive-type image display device requires a backlight, side light, or the like. Thus, when the reflective-type image display device is used for the display panel, lower power consumption can be achieved than a case of using the transmissive-type image display device. Therefore, the reflective-type image display devices have actively been applied to portable devices and the like in these days.
However, in such case where the reflective-type image display device is used, the external light is reflected as if it is reflected by a mirror plane when the reflection plate has a flat-face shape. Therefore, there is such an issue that a figure of the light source such as a fluorescent light is reflected so that the display quality is deteriorated. Further, only the light making incident from specific angles with respect to the viewer becomes contributed to the display, so that the use efficiency of the external light is deteriorated.
In order to improve such issues, Japanese Unexamined Patent Publication 08-184846 (Patent Document 2) proposes a technique which provides uneven shapes on a reflection plate. FIG. 31 shows a structural example of the reflection plate having the uneven shapes. According to Patent Document 2, an organic film is provided on a lower layer of a reflection plate 41, and unevenness is provided on the surface of the organic film to form uneven shapes 41a on the surface of the reflection plate 41. With the uneven shapes, external light making incident from a specific direction is reflected by being diffused towards various directions. Further, external light making incident from various directions is reflected also to the direction of the viewer. As a result, reflection of the figure of the light source can be prevented, thereby making it possible to utilize the external light of various angles for the display.
Further, Japanese Unexamined Patent Publication 2004-280079 (Patent Document 3) proposes a stereoscopic image display device that has a reflecting function. FIG. 33 shows a schematic illustration of the reflective-type stereoscopic image display device depicted in Patent Document 3. Further, as shown in FIG. 32, a plurality of display units each having a pixel for displaying a right-eye image and a pixel for displaying a left-eye image are arranged in matrix in a display panel 2.
A lenticular lens 3 is disposed in the front of the display panel 2, and it has convex shapes formed repeatedly on its surface so as to have a function of distributing the light emitted from each of the pixels towards the left and right directions connecting between the pixel for displaying the left-eye image and the pixel for displaying the right-eye image within each of the display units (see FIG. 32 and FIG. 33). The reflection plate 4 reflects the external light towards the display panel, and the above-described uneven shapes 41 are formed on the surface of the reflection plate 4. A focal distance f of the lens is different from a distance HR between the surface of the reflection plate 4 and the vertex of the lens.
In the reflective-type stereoscopic image display device shown in FIG. 33, the light converged by the lenticular lens 3 has a specific dimension on the surface of the reflection plate 4. Thus, it is reflected at a plurality of kinds of tilt angles, e.g., reflected at the slopes, the flat parts, and the like of the uneven shapes, so that the reflected light travels towards directions at various angles. This makes it possible to prevent deterioration in the luminance that is caused by the uneven shapes. That is, this is a method which isolates the position of lens focal point from the position of the reflection plate to shift the focal point of the enlarged area so as to blur the distributed images. This technique is referred to as “defocus effect” in this Application hereinafter.
As described above, the theoretical contents of the above-described stereoscopic image display device and the reflective-type flat display device using the lenticular lens 2 are well known, and technical investigations of the reflective-type/transflective-type stereoscopic image display devices applying those also have actively been conducted in various related fields in these days.
The inventors of the present invention have eagerly conducted studies so as the achieve a display device capable of performing stereoscopic image display with reflective display and to achieve low power consumption by combining the above-described stereoscopic image display device and the reflective/transflective-type flat display device. As a result, new issues as follows have become apparent.
First, the reflective display area within the pixel in the reflective/transmissive type display panel is disposed at a constant position within each pixel for keeping the uniformity of the reflective display. Thus, in the display panel where the pixels are arranged in matrix, the reflective display areas are periodically arranged according to the periodicity of the pixel matrix. In particular, data lines, scanning lines, and the like are provided in the boundary areas between the neighboring pixels in many cases, so that those areas become non-reflective display areas. Further, when there are transistors (TFTs), light is shielded with the black matrix, thereby forming the non-reflective display areas.
Therefore, as shown in FIG. 34, for example, when an image distributing device such as a lens is disposed by corresponding to two pixels (left-eye pixel 51, right-eye pixel 52) of the display panel 2, there may be a viewing area from which the reflective display cannot be visually recognized if a non-reflective display area 70 within the pixel is enlarged. This results in deteriorating the visibility of the reflective display.
Further, when the reflection plates within the pixels are arranged regularly according to the pixel layout even if the reflective display areas are in the lens focal positions partially, the light passing through the pixels is separated to the image distributing directions. Thereby, display unevenness caused by the reflection plates is to be visually recognized only at a specific viewing position. In addition, the pixels are enlarge-displayed by the effect of the lens, so that the display unevenness may be enlarge-displayed as well.
That is, in a stereoscopy viewable area that is originally supposed to have uniform luminance, there may be generated an area with deteriorated luminance depending on the viewing positions. In that case, when the viewing position is changed, the display becomes dark at a position where the luminance is deteriorated, and a dark-line pattern may be observed in some cases. Further, the quality of the stereoscopic image is deteriorated because of the unevenness of the luminance.
In order to decrease the display unevenness, there is considered a method of blurring the distributed images by shifting the focal points of the enlarged areas (Patent Document 3). However, there are still following issues remained, even when the technique depicted in Patent Document 3 is applied. That is, the images become blurred since the focal points are shifted, even though the deterioration of the luminance is decreased by the defocus effect. Therefore, the image separating performance becomes deteriorated while blurring the image, so that the display quality of the stereoscopic image becomes deteriorated.
Further, when there is shift generated at the time of mounting the image distributing optical devices on the flat display panel, the focal point positions on the display elements are shifted from the designed layout. Thus, the defocus effect is greatly reduced. Furthermore, when there is deflection and thermal contraction generated in the image distributing optical device, it is not possible to obtain a uniform defocus effect on the plane. Therefore, the defocused images are displayed as unevenness, which largely deteriorates the display quality of the stereoscopic images.